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New Approaches to Understand Constraint Effects on the Onset of Upper Shelf Temperature in a Reactor Pressure Vessel Steel
[Thesis]. Manchester, UK: The University of Manchester; 2013.
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Abstract
The Reactor Pressure Vessel (RPV) is a critical pressure boundary component in a nuclear power facility and is made of ferritic steel, which exhibits a change in its failure mechanism of from brittle cleavage to ductile fracture with increasing temperature. The RPV is designed to operate at temperatures where the material is ductile under normal operating conditions and this is achieved by ensuring that when stressed, the RPV temperature is above the Onset of Upper Shelf Temperature (OUST). The OUST is defined as that temperature where the steel has a combined 5% probability of cleavage and a 50% probability of ductile fracture, measured using high constraint fracture toughness specimens. However, the OUST will reduce for defects that have a lower level of crack-tip constraint, e.g. shallow cracks that exhibit reduced levels of stress triaxiality. It is therefore important to develop new understanding of constraint effects on the OUST for RPV steels. This thesis is aimed at developing a new methodology with which to assess the combined influence of constraint, microstructure and fracture mechanism on the fracture toughness of RPV steels at temperatures near the OUST. A Master Curve statistical analysis, and associated metallography, of the so-called Euro RPV material was performed. Subsequently, a stress contour-based approach for studying constraint effects on fracture was assessed and used within 3D design analyses of deep and shallow-cracked fracture mechanics specimens loaded in four-point bending: (a) a single specimen containing two offset cracks, (b) two single crack specimens loaded in series (i.e. one on top of the other), and (c) two single crack specimens loaded in parallel (i.e. side by side). The stress contour-based approach was shown to be a valid approach for assessing constraint effects. The single specimen containing offset cracks was shown to be the most favourable experimental approach for the following reasons. Two specimens loaded in series limit to the maximum applied displacement and exhibit frictional effects at the supporting rollers. Two specimens loaded in parallel with slightly different crack lengths will have different compliances. With appropriate design, a single specimen containing two offset cracks can be used to study the constraint effects on OUST and the interaction between cleavage and ductile fracture mechanisms. This experimental approach, combined with an analysis of the associated crack-tip stress fields can provide a new approach for studying the influence of constraint on the OUST in RPV steels.